The present invention generally relates to medical cardiac pacemakers and more specifically relates to a system and method for optimizing activity threshold in activity based rate adaptive cardiac pacemakers of the type which respond to the patient's metabolic demand and varies the pacing rate in accordance therewith.
Early cardiac pacemakers were asynchronous in operation, providing stimulating pulses to the heart at a fixed rate independent of the physiologic demand of the patient. In recent years, pacemakers which measure the metabolic demand for oxygen and vary the pacing rate in response thereto have become widely available. One modern method employed for measuring the need for oxygenated blood includes measurement of patient physical activity by means of an appropriate sensor. Generally, a rate responsive pacemaker which is responsive to patient physical activity includes a sensor which produces an output that varies between a maximum sensor output level and a minimum sensor output level and provides for a pacing rate which typically varies between a selectable lower pacing rate and an upper pacing rate. Such a pacemaker which utilizes a piezoelectric transducer, is disclosed in U.S. Pat. No. 4,485,813, issued to Anderson, et al., and assigned to Medtronic, Inc.
It has become common practice in recent years to provide programmable parameters in order to permit the physician to select and adjust the desired parameters to match or optimize the pacing system to the heart patient's physiologic requirements in an effort to minimize patient problems and to prolong or extend the useful life of an implanted pacemaker. Such systems are based upon utilizing a sensor derived variable that is an indicator of the patient's true metabolic and physiologic needs.
Activity based rate adaptive pacemakers typically have a programmable activity threshold parameter for adjusting the sensitivity of the circuitry to signals from the activity sensor. Historically, activity threshold parameters in activity based rate adpative pacemakers have been manually programmed and adjusted or optimized in an ad hoc iterative process. Often, because the process is difficult and lengthy, such parameters are not optimized, but left at nominal shipping values. Physicians using activity based rate adaptive pacemakers may have difficulty setting the activity threshold to an optimal setting since this parameter is device specific and there is a marked proliferation of activity based rate adaptive pacemakers by both Medtronic and other companies. Such a parameter will always tend to be device specific more than patient specific, so the physician has no inherent expertise in setting it optimally.
An activity based rate adaptive pacemaker which is capable of automatically and continuously determining and adjusting the activity threshold to an optimal setting would both significantly reduce programming time commitment by the physician and insure that activity threshold optimized at any single point in time will continue to remain optimal. One method for dealing with setting the activity threshold to an optimal setting for a given patient is disclosed in the auto set up idea disclosures included in U.S. patent application Ser. No. 07/567,372 entitled Rate Responsive Pacemaker and Method for Automatically Initializing the Same, filed in the name of Roline, et al., on Aug. 14, 1990, and assigned to Medtronic, Inc. The auto set up feature disclosed by Roline is desirable, but has limitations. It requires significant programming timing commitment by the physician. Also, it is unlikely that activity threshold optimized at any single point in time will remain optimal. The location of the pacemaker (subtle movements in the pocket), the IPG pocket characteristics, the patient's tissue composition (gaining/losing weight), the types of patient activities, and vibrational environmental factors may all impact how the activity threshold should be set. While Roline et al. constitutes an improvement over the conventional methods, it has not proven to be completely satisfactory in addressing and resolving the optimization problems associated with the optimization process.
It is an object of this invention to overcome the disadvantages and drawbacks of the prior art and to provide a method of activity-based rate-adaptive pacemaking which significantly reduces programming time commitment by the physician or repeated patient sessions in determining and adjusting the activity threshold to an optimal setting, and in which practice the activity threshold optimized at any single point in time will continue to remain optimal.
It is a further object of this invention to provide a system for practicing the above said method.